Oscillator circuits



Patented F eb. 26, 1946 OSCILLATOR CIRCUITS Winfield R. Koch, Haddonield, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application .l une 15, 1942, Serial No. 447,008

(Cl. Z50-20) 4 Claims.

'to two-terminal circuit oscillators which can be readily synchronized with an applied voltage.

As is wel1 known to those skilled in the art, the early screen grid tubs exhibited a negative resistance in the screen circuit by virtue of the phenomenon of secondary emission. Such a screen grid tube circuit usually had its plate at a lower positive potential than the screen grid, and the circuit was known as a dynatron circuit. Such dynatron circuits were particularly employed to produce oscillators of high frequency. I have found that a similar characteristic can be secured with a tube of the pentagrid type. In this type of tube there are usually located ve control grids between the cathode and anode; the middle grid being located between a pair of positive shielding grids.

It may, therefore, be stated that it is one of the main objects of my present invention to utilize a pentagrid type of tube in a controlled dynatron oscillator circuit.

Another object of the invention is to employ the third, or middle, grid of a pentagrid tube in the negative resistance circuit, and, because of the screening action provided by the second grid relative to the cathode, to control oscillations at radio frequency without undesirable capacity coupling.'

Another important object of the invention is to utilize a pentagrid tube, connected as a twoterminal oscillator, in a frequency divider network of a frequency modulated carrier wave receiver.

Yet another object of the invention is to provide an electron oscillator utilizing a pentagrid type of tube whose middle grid has connected to it the resonant oscillator tank circuit, while the grid adjacent the cathode has supplied to it modulation signals.

Still other objects of the invention are to improve generally controlled dynatron oscillator circuits, and more especially to provide oscillatorcircuits of the dynatron type which are not only reliable and eicient in operation, but are economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; th'e invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circiut organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a dynatron oscillator circuit embodying th'e invention,

Fig. 2 illustrates the characteristic secured at l the middle grid of the oscillator tube of Fig. 1,

Fig. 3 illustrates another use of the invention in a frequency modulation receiver.

Referring to Fig. 1, the tube T schematically designates a pentagrid type of tube of the GSA? class. This type of tube comprises an electron emission electrode, such as a cathode K, and the anode electrode 6. In th'e electron stream flowing from cathode K to anode 6 there are located live control grids arranged in succession. Consider, rst, that the control grid l immediately adjacent the cathode is connected directly to the grounded cathode. The effect of the audio modulation source S will be considered at a later point. Grids 2 and l are connected together to a potential point which may h'ave a potential of approximately plus 100 volts. The direct current source for energizing the electrodes of the tube is omitted for the sake of simplicity.

The middle, or third, grid 3 is located between the posiitve electrodes 2 and ll. The grid 3 is connected to the high alternating potential side of th'e resonant circuit 'l which is tuned to the operating high frequency of the system. The circuit 1 has its low potential side connected to a point of positive potential of approximately plus 30 volts, the low side of circuit 'l being bypassed to ground for high frequency currents. It will be understood that circuit 'l exhibits the negative resistance of the oscillator circuit. The tank circuit 1 may be tuned to an intermediate frequency, a radio frequency, or an ultra-high the megacycle range.

The grid 5 adjacent the anode 6 is shown as grounded. It will be understood that grids 2`and 4 provide an electrostatic shielding field around the intermediate grid 3. The anode is connected to the plus 100 volts potential point. Still assuming that the modulation source S is not incircut with grid l, Fig. 2 shows the characteristic of the intermediate grid in the circuit of Fig. 1.

The characteristic of Fig. 2 is secured by plotting the direct current potential of grid 3 against direct current flow in the circuit connected to grid 3. It will be observed that the characteristic includes a substantial negative resistance portion. According to my present invention, the

0 circuit of Fig. l is operated so as to have the circuit 1 connected to grid 3 exhibit the negative resistance as indicated by the characteristic curve of Fig. 2. It will be seen that unlike the earlier dynatron circuits of the screen grid type, the intermediate grid is `at a lower positive potential than the plate cf the tube. The plate in this case is at the same, or more positive, potential than the screen, but the electrode exhibiting the negative resistance is in each case less positive than the screen. In the present circuit, emitted frequency in electrons are drawn to either the preceding electrode, grid 2, or to the following electrode, grid 4, instead of to only the preceding grid as in previous circuits. This results in better distribution of the heat loss of the electrodes and in a lower negative resistance, which will produce oscillations with circuits of lower dynamic resistance.

By applying audio modulation signals to grid I, amplitude modulation of the radio frequency oscillations produced in circuit 1 may be secured.

The modulated radio frequency energy could be taken from the tuned circuit through suitable coupling impedances. It could ralso be taken from the plate,` screen, or cathode circuits by inserting a coil or resistor to give a voltage drop which can lbe .applied tothe utilization lcircuit. Theparticular advantages of the modulator of Fig. 1 are the simple tuned circuit without tick-lors, taps, etc., and resulting good stability.

In Fig. 3 I have lshown another use for the present invention. The controlled dynatron :osycill'ator is here employed as :a frequency divider Aof the `type disclosed by G. L. Beers in his applituition Serial No. 430,588, zii-led February l2, 1942. The frequency divider in that application is empioyed h1 a frequency modulated ycarrier wave (FM) receiver. The frequency divider 'in 'the Beers application is an Aoscillator fed by the intermed-late frequency .(I. F.) network of a super- .ire'terodyne receiver, the oscillator operating at .a sub-multiple .harmonic tand being adapted to `:be locked in with the applied I. F, signal energy. The function of the oscillator Ais to reduce the 'mean lor mid`band,frequency of the applied I. F. energy to a frequency value which is substantially lower than the .mean -frequency of the -applied 'L energy. Concurrently, .the frequency deviation range tof the fapplied I. i". :energy .is reduced proportionately withthe reduction -of the mean .frequency value. In 'the Beers `circuit 'a .tour-'terminal :oscillator Icircuit is utilized. `By employing .my present invention it is possible to secure the same result witha two-terminal oscil- 4lator circuit with resulting economy iin -cost -of receiver construction.

'Considering Fig. 3, thenumeral It denotes the clocked-'in oscillator vtube of 'the pentagrid type.

The cathode lII .is grounded. The middle, or intermediate, grid I2 is connected to a `source of vplus 30 `volts through the coil -I3 -of the resonant circuit Il. .The secondandfourth `grids are again v'connectedin commento the 'plus 10U volts potential point, 'and .the-anode t5 is similarly connected to vthat .positive potential point. The first -grid :i6 is connected to thehigh potential side-of the resonant secondary 'circuit I :'l of fthe I. F. input transformer t8. The `resonant primary circuit I9 of transformer I8 is arranged in the plate 4circuit of va preceding I. iF. amplifier. I'he low .potentialsi'delof-secondary circuit I7 -is-'connected tolground 1byva resistor 2li which is bypassed for I. currents. Direct current `voltage developed 'aoross'resistorizu :is utilized for automatic -volume fcontrol (AVC), and the AVC lead '21, which include's the usual lter resistor, is `employed `for feeding the AVC "bias -to the 'signal grids of the prior signal transmission tubes.

Immediatelyabove the `.transformer I 8 .I have *depictedthe ltyp'e of response characteristic which isl'de'ally 'desired of the I. F, transformer. It will be observed .that the symbol En is .given `a value of i413 mega'cycles (mcl)- This signifies `that .the mean, or center, .frequency of the ,1. F.-energy 'it `transformer I8 'has .a value lof 4.3 .-mc. The

I. F. signal waves. I A sistan'ce yof vlgrid I2 Willloe obtained during -ilve transformer has a pass band width of approximately kilocycles (kc). In this way the transformer is able eiiiciently to pass frequency deviation up to 75 kc. on either side of the Fc value. The circuit I4 is tuned to a mean, or center, frequency of 860 kc. The pass band width of circuit I4 need only be 30 kc., since the action of the oscillator circuit is concurrently to reduce Fc and the frequency deviation range by the same proportionate integer. This action has been explained 'in detail 'in the aforesaid Beers application, and it is not believed necessary to describe 'it .in detail in this application. The action of the locked-in oscillator is to frequency divide the applied FM energy; proportionately reduce the frequency deviation range; and also secure limiting of any amplitude modulation .effects which may :exist on'the FM vwave energy.

The FM wave energy developed across circuit le may be applied to any well known form .of fdiscriminator-.rectier network. `For example, the circuit M may be :the primary 'cir-.cuit :of :the well known discriminatori-rectifier disclosed by S. Seeley `in his S..Patent 2,121,103, granted June 2i, .1938. 0n the other hand :there may :be femployed a -discriminator-reotier network of the type vdisclosed the aforesaid Beers application. The function of the -discriminator-rectier .network is to derive the audio modulation signals from the FM wave energy developed Yacross resistor circuit I4.

Comparing the .locked-in yoscillator .of Fig. 3 with the vgeneralized modulation circuit rshown in Fig. 1, it will be seen that the FM energy applied to grid I6 in Fig. 3 functions as the appl-iedcon- .trolling voltage, whereas the negative resistance .effect is secured in circuit AHI connected to grid sI-2. Here the negative resistance `presented by the tube is applied across 1the resonant input lcircuit. M of the following .discriminator network. Oscillations at approximately the .center .frequency .of the discriminator will result, if the t-i-mpedanoerof ,the -discrirninator is higher than the negative resistance of the tube. y A

Since theelectron stream pausing fthe-secondary emission from grid I2 is controlled by the voltage of .grid 16 the FM wave energy applied to the grid .l5 will be vvery eiective in locking in the oscillations, or in keeping them in step with :the signal energy as it is frequency modulated. .It will benoted that .circuit .I II is tuned to asub-fmultiple harmonic of the vapplied FM signal frequency, and that the .FM wave energy is controlled at za sub-multiple harmonic by the applied 4signal fenorgy. With the ordinary type of oscillator, ,which requires a tickler-coiLanother tunedlcircuit is required ahead .of the discriminator network for the tickler .to vcouple to. However, by using the present invention the eXtra tuned circuit .can be omitted with resulting .savings in .thecost .of v.the

receiver.

The grid 'I2 exhibits a negativeresistance which causes oscillations at the frequency of 'the tuned circuit, or one-fifth thato'f the .appliedl F. signal. Due to 'the self-biasing `action of the input grid circuit, current will flow `througlfi grid I6 'only during Athe most positiveportions of the applied Therefore, the negative Vresmall portions of each 860 kc. oscillation. The

`phase ofthe current :pulseawhen 'the applied frequency is exactly five times the natural Lfrequency of oscillations of the tuned circuit, `fwill bearra .definite lrelation to the phase of ithe 'currents in the tuned circuit. VIf the :applied -frethe same effect as changing the reactance across the tuned circuit, causing it to tend to shift in frequency in such a direction that over a limited range the oscillations of the tuned circuit will always be maintained at exactly one-fifth the frequency applied. That is, the current from the grid becomes effectively reactive as well as resistive, thus changing the frequency.

Instead of a simple tuned circuit connected to grid l2, the discriminator circuit may be used. This type of circuit may exhibit a high resistance over a broader band of frequencies than a single tuned circuit. In addition, the reactance offered to the gridl2 may change much less with frequency than that of a single tuned circuit in the region of oscillations. In fact,- the reactance vs. frequency curve may slope in the opposite direction from that of single circuit, depending on the coupling and damping of the circuits. It is" easily seen, therefore, that inorder to secure lock-in over a denite frequency range, the reactance of the discriminator circuit should be small throughout the range, so that the reactive component of the grid current in grid l 2 will have a controlling effect.

Since the locally generated oscillations are always a sub-multiple of the applied I. F. signal,

a given percentage change in frequency of the applied I. F. signal will result in the same percentage change in frequency of the generated oscillations. That is, when the applied I. F. signal is 4,300+'75=4375 kc., the locked-in oscillator will be 4375+5=875=860+15 kc. The deviation in each case is 1.75%, but the '75 kc. deviation has been reduced to 15 kc. deviation.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as s'et forth in the appended claims.

What 1 claim is:

1. In an oscillator circuit of the dynatron type, a tube provided with an electron emission electrode, a plate electrode, a control grid located in the electron stream of said tube between said emission electrode and plate, a resonant output circuit tuned to a desired high frequency, means for connecting said grid to said resonant circuit whereby said grid acts as the output electrode of the circuit, means for establishing said grid at a positive direct current potential, means for establishing said plate electrode at a substantially higher positive direct current potential, a second control grid located between said emission electrode and said rst grid for providing an electrostatic shielding eld, means for establishing said second grid at a positive potential which is higher than the potential of said rst grid, a third grid located between the emission electrode and said second grid, a source of frequency modulated carrier Wave energy connected to said third grid, said resonant circuit connected to the first grid being tuned to a submultiple harmonic frequency of the mean frequency of said frequency modulation energy and a wave energy utilizing circuit coupled to solely said resonant circuit.

2. In a frequency divider network, a tube provided with an electron emission electrode,plate electrode, a control grid located in the electron stream of said tube between said emission electrode and plate, a resonant output circuit tuned to a desired high frequency, means for connecting said grid to said resonant circuit whereby said grid acts as the output electrode of the circuit, means for establishing said grid at a positive potential, means for establishing said plate electrode at a substantially higher positive potential, an input grid located between the emission electrode and said control grid, a source of angular velocity-modulated carrier. wave energy connected to said input grid, said resonant circuit connected to the controlgrid being tuned to a sub-multiple harmonic frequency of the center frequency of said modulated carrier energy and a modulated wave energy utilization circuit coupled to solely said resonant circuit.

3. In a frequency modulation receiver, a frequency divider of the dynatron type comprising a tube provided with an electron emission electrode, a plate electrode, a control grid located in the electron stream of said tube between said emission electrode and plate, a resonant circuit output, means for connecting said grid to said resonant circuit whereby said grid acts as the output electrode, means for establishing said grid at a positive potential, means for establishing said plate electrode at a substantially higher positive potential, a second control grid located between said emission electrode and said first grid for providing an electrostatic shielding field, and means for establishing said second grid at a positive potential which is higher than the potential of said first grid, a third grid located between the emission electrode and said second grid, a source of frequency modulated carrier wave energy connected to said third grid, said resonant circuit connected to the rst grid being tuned to a sub-multiple harmonic frequency of the center frequency of said frequency modulation energy and a modulated wave energy utilizing circuit connected to solely said resonant circuit.

4. In a frequency divider network, a tube provided with anl electron emission electrode, a plate electrode, a control grid located in the electron stream of said tube between said emission electrode and plate, a resonant output circuit tuned to a desired high frequency, means for connecting said grid to said resonant circuit whereby said grid functions as the output electrode of the divider network, means for establishing said grid at a positive potential, means for establishing said plate electrode at a substantially higher positive potential, said plate electrode being free of any high frequency circuit, an input grid located between the emission electrode and said control grid, a source of angular velocity-modulated carrier wave energy connected to said input grid, said resonant output circuit connected to the control grid being tuned to a sub-multiple harmonic frequency of the center frequency of said modulated carrier energy, and said source being a resonant input circuit tuned to said center frequency and having a pass band substantially wider than the pass band of the resonant output circuit. k

WINFIELD R. KOCH. 

